For 4 years we are in the market, I have constantly tried to find any pattern on how some items go really good while others do not even pass the 100 sales threshold both being almost 1:1 similar (at least from technical view, apparently the design is somewhat different). So I hate to say that but there’s no logic (or at least I couldn’t found one) and will agree to the authors that first comes luck (actually first 3 places perhaps ) and then the design quality, ease of use, level of support provided, etc…

To our opinion, demo website is the most important thing! Of course you will need a great design and bunch of cool options, but all this becomes useless if buyers are not able to see good presentation of your features. Use demo website not just as an live example but as a good overview of your entire work.

Also, I see obvious bugs on demo websites from time to time. Check the responsiveness and cross-browser, at least on your demo pages.

2. Compatibility:

Avoid custom stuff inside the theme and tend to make it as WordPress friendly as possible. Make the users see that they can easily switch from their old theme to your theme. Approximately speaking, 30% of our customers use our themes to switch from existing themes. Which means that if your theme relies on custom stuff you loose lots of customers.

3. Transparent and “useful” design (think as a customer)

Just because you are different (unique) it doesn’t mean you are useful. Just because you have learn new css technique on codrops or some other fancy website it doesn’t mean that you need to overload your theme with new “shiny disco ball” effects. I truly believe that so called “cool” design effect will rather refuse customers than attract them. Offer the people what they are used to get.

Try to learn from real successful websites, not only from themes here on ThemeForest. You are creating real thing for real people, you are not on some design competition/exhibition.

4. Make it loads fast

Nobody wants a theme which crawls. If people have to wait more than 2 seconds, they will mostly close the window and search for another one. Also, have in mind that not all the people live in the USA with hundreds of MBs internet connection speed. Try from 3G.

5. Luck

Yes, you will need a bit of luck, but only if you are not sure what you are doing.